The Australian bushfires around the turn of the year 2020 generated an unprecedented perturbation of stratospheric composition, dynamical circulation and radiative balance. Here we show from ...satellite observations that the resulting planetary-scale blocking of solar radiation by the smoke is larger than any previously documented wildfires and of the same order as the radiative forcing produced by moderate volcanic eruptions. A striking effect of the solar heating of an intense smoke patch was the generation of a self-maintained anticyclonic vortex measuring 1000 km in diameter and featuring its own ozone hole. The highly stable vortex persisted in the stratosphere for over 13 weeks, travelled 66,000 km and lifted a confined bubble of smoke and moisture to 35 km altitude. Its evolution was tracked by several satellite-based sensors and was successfully resolved by the European Centre for Medium-Range Weather Forecasts operational system, primarily based on satellite data. Because wildfires are expected to increase in frequency and strength in a changing climate, we suggest that extraordinary events of this type may contribute significantly to the global stratospheric composition in the coming decades.
The 2019/2020 Australian wildfires generated a smoke cloud that organized itself into a persistent vortex structure and ascended to 35 km altitude through solar heating, according to satellite tracking.
We use a combination of spaceborne instruments to study the unprecedented stratospheric plume after the Tonga eruption of 15 January 2022.
The aerosol plume was initially formed of two clouds at 30 ...and 28 km, mostly composed of submicron-sized sulfate particles, without ash, which is washed out within the first day following the eruption.
The large amount of injected water vapour led to a fast conversion of SO2 to sulfate aerosols and induced a descent of the plume to 24–26 km over the first 3 weeks by radiative cooling.
Whereas SO2 returned to background levels by the end of January, volcanic sulfates and water still persisted after 6 months, mainly confined between 35∘ S and 20∘ N until June due to the zonal symmetry of the summer stratospheric circulation at 22–26 km.
Sulfate particles, undergoing hygroscopic growth and coagulation, sediment and gradually separate from the moisture anomaly entrained in the ascending branch Brewer–Dobson circulation.
Sulfate aerosol optical depths derived from the IASI (Infrared Atmospheric Sounding Interferometer) infrared sounder show that during the first 2 months, the aerosol plume was not simply diluted and dispersed passively but rather organized in concentrated patches. Space-borne lidar winds suggest that those structures, generated by shear-induced instabilities, are associated with vorticity anomalies that may have enhanced the duration and impact of the plume.
The article presents new high-quality continuous stratospheric aerosol observations spanning 1994–2015 at the French Observatoire de Haute-Provence (OHP, 44° N, 6° E) obtained by two independent, ...regularly maintained lidar systems operating within the Network for Detection of Atmospheric Composition Change (NDACC). Lidar series are compared with global-coverage observations by Stratospheric Aerosol and Gas Experiment (SAGE II), Global Ozone Monitoring by Occultation of Stars (GOMOS), Optical Spectrograph and InfraRed Imaging System (OSIRIS), Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), and Ozone Mapping Profiling Suite (OMPS) satellite instruments, altogether covering the time span of OHP lidar measurements. Local OHP and zonal-mean satellite series of stratospheric aerosol optical depth are in excellent agreement, allowing for accurate characterization of stratospheric aerosol evolution and variability at northern midlatitudes during the last 2 decades. The combination of local and global observations is used for a careful separation between volcanically perturbed and quiescent periods. While the volcanic signatures dominate the stratospheric aerosol record, the background aerosol abundance is found to be modulated remotely by the poleward transport of convectively cleansed air from the deep tropics and aerosol-laden air from the Asian monsoon region. The annual cycle of background aerosol at midlatitudes, featuring a minimum during late spring and a maximum during late summer, correlates with that of water vapor from the Aura Microwave Limb Sounder (MLS). Observations covering two volcanically quiescent periods over the last 2 decades provide an indication of a growth in the nonvolcanic component of stratospheric aerosol. A statistically significant factor of 2 increase in nonvolcanic aerosol since 1998, seasonally restricted to late summer and fall, is associated with the influence of the Asian monsoon and growing pollution therein.
The Asian monsoon anticyclone (AMA) represents one of the wettest regions in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapour. While the AMA wet ...pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from aboard the high-altitude M55-Geophysica aircraft deployed in Nepal in summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modelling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of the AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in the AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within the AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within the AMA for the transport of water into the stratosphere.
During the StratoClim Geophysica campaign, air with total water mixing ratios up to 200 ppmv and ozone up to 250 ppbv was observed within the Asian summer monsoon anticyclone up to 1.7 km above the ...local cold-point tropopause (CPT). To investigate the temporal evolution of enhanced water vapor being transported into the stratosphere, we conduct forward trajectory simulations using both a microphysical and an idealized freeze-drying model. The models are initialized at the measurement locations and the evolution of water vapor and ice is compared with satellite observations of MLS and CALIPSO. Our results show that these extremely high water vapor values observed above the CPT are very likely to undergo significant further freeze-drying due to experiencing extremely cold temperatures while circulating in the anticyclonic “dehydration carousel”. We also use the Lagrangian dry point (LDP) of the merged back-and-forward trajectories to reconstruct the water vapor fields. The results show that the extremely high water vapor mixed with the stratospheric air has a negligible impact on the overall water vapor budget. The LDP mixing ratios are a better proxy for the large-scale water vapor distributions in the stratosphere during this period.
Deep convection overshooting the lowermost stratosphere is well known for its role in the local stratospheric water vapour (WV) budget. While it is seldom the case, local enhancement of WV associated ...with stratospheric overshoots is often published. Nevertheless, one debatable topic persists regarding the global impact of this event with respect to the temperature-driven dehydration of air parcels entering the stratosphere. As a first step, it is critical to quantify their role at a cloud-resolving scale before assessing their impact on a large scale in a climate model. It would lead to a nudging scheme for large-scale simulation of overshoots.
A direct-detection Rayleigh–Mie Doppler lidar for measuring horizontal
wind speed in the middle atmosphere (10 to 50 km altitude) has
been deployed at Observatoire de Haute-Provence (OHP) in southern
...France starting from 1993. After a recent upgrade, the instrument gained the
capacity of wind profiling between 5 and 75 km altitude with
vertical resolution up to 115 m and temporal resolution up to
5 min. The lidar comprises a monomode Nd:Yag laser emitting at
532 nm, three telescope assemblies and a double-edge
Fabry–Pérot interferometer for detection of the Doppler shift in the
backscattered light. In this article, we describe the instrument
design, recap retrieval methodology and provide an updated error
estimate for horizontal wind. The evaluation of the wind lidar
performance is done using a series of 12 time-coordinated
radiosoundings conducted at OHP. A point-by-point intercomparison
shows a remarkably small average bias of 0.1 m s−1 between
the lidar and the radiosonde wind profiles with a standard deviation
of 2.3 m s−1. We report examples of a weekly and an hourly
observation series, reflecting various dynamical events in the middle
atmosphere, such as a sudden stratospheric warming event in January
2019 and an occurrence of a stationary gravity wave, generated by the
flow over the Alps. A qualitative comparison between the wind profiles
from the lidar and the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System is also
discussed. Finally, we present an example of early validation of the
European Space Agency (ESA) Aeolus space-borne wind lidar using its ground-based predecessor.
We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics ...was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m−2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources.
In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, ...and hydration–dehydration of the tropical tropopause layer.
Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (<5 ppmv) and a challenging low temperature–pressure environment.
The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer).
Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence.
In clear-sky UTLS conditions (H2O<10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %.
Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %.
Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively.
This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.
The 15 January 2022 eruption of the Hunga volcano (Tonga) generated a rich spectrum of waves, some of which achieved global propagation. Among numerous platforms monitoring the event, two ...stratospheric balloons flying over the tropical Pacific provided unique observations of infrasonic wave arrivals, detecting five complete revolutions. Combined with ground measurements from the infrasound network of the International Monitoring System, balloon‐borne observations may provide additional constraint on the scenario of the eruption, as suggested by the correlation between bursts of acoustic wave emission and peaks of maximum volcanic plume top height. Balloon records also highlight previously unobserved long‐range propagation of infrasound modes and their dispersion patterns. A comparison between ground‐ and balloon‐based measurements emphasizes superior signal‐to‐noise ratios onboard the balloons and further demonstrates their potential for infrasound studies.
Plain Language Summary
The eruption of the Hunga volcano on 15 January 2022 was one of the most powerful blasts of the last century. This fast and strong perturbation of the atmosphere triggered atmospheric waves which were followed around the world multiple times. Here, we use records of sound waves emitted by the eruption from two balloons flying at about 20 km altitude over the Pacific combined with ground stations around the volcano to help characterize the event and its scenario. Due to weak relative wind and turbulence, the sounds on the balloon are generally clearer than on the ground, demonstrating the potential of high‐altitude measurements for extreme events.
Key Points
Comparison between balloon‐borne and ground‐based observations of infrasound waves triggered by the January 2022 Hunga eruption
Eruption sequence from infrasound in broad agreement with plume top height evolution
Benchmark for long‐range monitoring of infrasound from large explosive sources using stratospheric balloon observations
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